1. Field of the Invention
The present invention relates generally to a method and apparatus for providing a positive pressure therapy particularly suited to treat Cheyne-Stokes respiration and other breathing disorders commonly associated with congestive heart failure.
2. Description of the Related Art
Congestive heart failure (CHF) patients commonly suffer from respiratory disorders, such as obstructive sleep apnea (OSA) or central apneas. Another such respiratory disorder CHF patients often experience during sleep is known as Cheyne-Stokes respiration. FIG. 1 illustrates a typical Cheyne-Stokes respiration (CSR) pattern 30, which is characterized by rhythmic waxing periods 32 and waning periods 34 of respiration, with regularly recurring periods of high respiratory drive (hyperpnea) 36 and low respiratory drive (hypopnea or apnea) 38. A typical Cheyne-Stokes cycle, generally indicated at 40, lasts about one minute and is characterized by a crescendo (arrow A), in which the peak respiratory flow of the patient increases over several breath cycles, and decrescendo (arrow B), in which the peak respiratory flow of the patient decreases over several breath cycles. The typical Cheyne-Stokes cycle ends with a central apnea or hypopnea following the decrecendo phase. Apneas, hyperpneas, and the abnormal change in the depth and rate of breathing often cause arousals and, thus, degrades sleep quality. This disruption in sleep, as well as the periodic desaturation of arterial oxygen (PaO2), caused by the CSR cycle stresses the cardio-vascular system and specifically the heart.
The earliest treatment for CSR involved stimulating the respiratory drive by administering Theophyline, caffeine, or 1-3% inspired carbon dioxide to the patient. Although sometimes effective in reducing CSR, the downside of these treatments, which increase the respiratory rate, is that the increase in respiratory rate proportionally increases cardiac and respiratory workload.
Recent work in the treatment of sleep apnea has included the use of a continuous positive airway pressure (CPAP) therapy in which a relatively constant positive airway pressure is delivered to the airway of a patient. Positive airway pressure therapy has been applied not only to the treatment of breathing disorders, such as OSA, but also has been used in the treatment of CHF. The effect of the CPAP therapy when used to treat CHF is to raise the pressure in the chest cavity surrounding the heart and allows cardiac output to increase.
Bi-level positive airway therapy has also been advanced in the treatment of sleep apnea and related breathing disorders. In bi-level therapy, pressure is applied alternately at relatively higher and lower prescription pressure levels within the airway of the patient so that the therapeutic air pressure is alternately administered at a larger and smaller magnitude. The higher and lower magnitude positive prescription pressure levels are known as inspiratory positive airway pressure (IPAP) and expiratory positive airway pressure (EPAP), respectively.
Some preliminary investigations reveal that cardiac output improves when patients are supported using bi-level pressure therapy. It has also been recognized that CSR can be treated by augmenting respiratory effort with pressure support when the CSR pattern is in hypopnea region 38. To accomplish this, it is known to use a ventilator or pressure support system to deliver machine triggered breaths during the hypopnea interval when the patient's own respiratory drive is reduced or not present. It is also known to treat CSR by decreasing the ventilatory efficiency when flow is in a hyperpnea region 36. For example, published PCT Appln. No. WO 00/45882 teaches using rebreathing during a hyperpnea region to reduce the patient's ventilatory effectiveness, much the same way a person hyperventilating is coached to breathe into a paper bag.
Yet another approach to providing therapy for the treatment of CSR is described in U.S. Pat. No. 6,532,959 (“the '959 patent”). According to the teachings of this patent, patients are provided with ventilatory support using a blower and mask. The system taught by the '959 patent determines a parameter referred to as “instantaneous ventilation”, which is derived by measuring the volume inspired and the volume expired over a short period of time, calculating the average of the two, and then dividing this result in half. This derived instantaneous ventilation is used to adjust the level of ventilatory support by comparing the instantaneous to a target volume that is determined from a long-term average of the patient's respiratory volumes, i.e., an average of the volumes of the last 1-2 minutes. In theory, the short-term instantaneous ventilation will be less than the long-term target during a hypopnea phase of the CSR cycle. As a result, the ventilatory support to the patient's respiration is increased. The opposite result will occur during the hyperpnea phase of the CSR cycle.
One disadvantage of the method of treating CSR taught by the '959 patent is that in many cases, the average value of the past respiratory volumes does not produce a target volume that will result in sufficient treatment of the hypopneas and apneas. CSR has a continuum of severity and, depending on the level of severity, the target volume will need to be adjusted to values other than the average of the last 1-2 minutes. Moreover, the CHF patient may have some degree of airway obstruction that must be treated for its own sake, but it also must be treated because these obstructive events appear to drive the CSR pattern as well. Therefore, a simple system that sets the target volume based on a long-term average of the past volumes does not address the interplay of obstructing airways and CSR. It should also be noted that periodic leg movements, prevalent in 60%-80% of CHF patients, are also suspected to drive the CSR pattern. The volume calculation used by the '959 patent is also prone to errors due to small bias errors in the estimated patient flow and to detecting the onset and termination of inspiration.
Another CSR treatment technique is disclosed in U.S. Pat. No. 6,752,151 (“the '151 patent”). This patent describes a CSR detection and treatment technique that monitors the peak flow in a pressure support system coupled to a patient to determine whether that patient is experiencing CSR. If so, the '151 patent teaches increasing IPAP, EPAP, or both to treat the CSR pattern. Detecting CSR based on the peak flow is believed to be more reliable than detecting CSR based on measured volumes, because the effect of an error in the estimated patient flow is always smaller in a peak flow determination than that in a volume calculation.
One embodiment of the variable positive airway pressure technique taught by the '151 patent teaches changing a pressure support level based on a comparison between a current peak flow and a target peak flow. The pressure support level (PS) is the difference between the IPAP and EPAP levels. The algorithm for changing the pressure support for a new breath (PS(k+1)) is given in the '151 patent as follows:PS(k+1)=PS(k)+Gain*(Target Flow−Qpk(k)),  (1)where: k is the index of the previous breath, PS(k) is the pressure support level for the previous breath, Gain is a factor that converts flow into pressure, Target Flow is the target peak flow, and Qpk(k) is the peak flow from the previous breath.
The '151 patent teaches adjusting the pressure support on a breath-by-breath basis such that the peak flow is at least as high as the target peak flow. The result is that pressure support increases when the flow is in the hypopnea region and decreases to zero while flow is in the hyperpnea region. The pressure support is synchronized to patient effort when present. During a central apnea, the '151 patent teaches delivering machine triggered breaths at a predetermined rate and duration.
The '151 patent further teaches adjusting the target flow based on the effectiveness of the pressure support therapy and determines the degree of pressure support intervention. More specifically, the following three parameters are monitored: 1) a CSR shape index, 2) a CSR severity index, and 3) a pressure support (PS) index. Based on these criteria, the target peak flow and/or the EPAP level are adjusted.
While the '151 patent teaches a robust and reliable technique for treating CSR, the present inventors recognized that there may be some shortcomings with this technique. For example, the '151 patent monitors the actual flow Qpk(k) in determining the pressure support and in analyzing the effectiveness of the CSR treatment. However, this actual peak flow may include anomalies that can introduce errors in the calculations performed by the device taught by this patent. In addition, the technique taught by the '151 patent for selecting the Target Flow may not maximize effectiveness in controlling the pressure support. Furthermore, the '151 patent does not adjust the pressure during a breath to ensure that the patient receives the necessary pressure or flow during each breath or to prevent the patient from receiving too high a pressure or flow during that respiratory cycle.